The present invention relates to an extruder apparatus for the extrusion of multi-layers of plastic resin and more particularly to an improvement in a multi-port selector plug for the feedblock portion of such apparatus.
At the present time, it is known that a number of layers of plastic resin may be formed into a flat or tubular film by co-extrusion. The various plastic resin layers may be of different thermoplastic resins and the layers may be different thicknesses. Each of the plastic resins, generally in the form of pellets, is individually melted and forced by pressure through the extruder apparatus and out of the orifice of its multi-manifold extrusion die to form a multi-layer laminate. The term "melt-lamination" is sometimes used to describe this process, and the lamination may be formed with, or without layers of adhesive, depending on the thermoplastic resins used in the lamination.
The die generally forms the multi-layer laminate into a flat sheet, a flat film or a tubular film. The multi-layer laminate is advantageous compared to a single layer of a uniform material, because each plastic resin layer of the laminate may be selected for its best property, such as a layer which is a barrier to oxygen, thereby forming a combination of layers having a combination of properties. In "co-extrusion" or "melt-lanination" the molten resins are formed into layers, within the extruder, and joined under pressure within the die to emerge as a single laminated material. The molten layers, within the feedblock and die, under proper flow conditions, exhibit laminar flow in which the molten layers are joined without intermixing.
A "multi-manifold" die system has an individual flow channel ("manifold") for each layer. Generally the laminar flow layers are brought together just above the exit orifice of the die. Each manifold may be provided with an adjustable flow restricter so that the uniformity of the layer's thickness may be adjusted.
It is also known that the relative order of the layers may be changed using a multi-port valve device, called a "Cloeren diverter plug" or "selector plug", without repiping the extruder, see "Coextrusion takes a giant step into the future", Modern Plastics, August 1983. The multi-port valve device consists of a plug in a barrel, the plug generally having surface channels and two or more channels crosswise to its axis. The plug may be rotated, for example by one-half turn, to line up each of its channels with one pair of exit and entrance ports. The plug may again be rotated to re-align its channels with different pairs of ports. The plug directs the molten plastic resins so that they flow in a selected order. For example, if the plastic resin layers of a co-extruded film are in the order A, B, C with B as an internal layer between outer layers A and C, the plug may be rotated so that the order becomes A, C, B with C becoming the inner layer.
However, the co-extrusion diverter plug presents a mumber of problems. First, leakage may occur between the layers, which results in defects in appearance. Secondly, the plug divider may become worn so that its channels do not exactly align with the various pairs of ports. This in a possible further cause of leakage and results in appearance defects in the finished film.